There has been active development of electrochromic (hereinafter, also abbreviated as “EC”) devices including electrochromic materials in which optical absorption properties, such as colored states and optical transmittances of materials, are changed by electrochemical redox reactions.
PTL 1 discloses an EC device in which a conductive polymer is formed on a transparent electrode and in which an electrolytic solution is enclosed between the electrode and a counter electrode. PTL 2 discloses a solution-phase EC device in which an electrolytic solution containing a low-molecular-weight molecule, such as viologen, dissolved therein is enclosed between a pair of electrodes.
For the conductive polymer described in PTL 1, an EC layer can be directly formed on the electrode by the electrolytic polymerization of a monomer. Known examples of the conductive polymer that forms the EC layer include polythiophene, polyaniline, and polypyrrole.
In the case where such a conductive polymer is electrochemically oxidized or reduced, the π-conjugated chain length of a main chain is changed, thereby changing the electron state of the highest occupied molecular orbital (HOMO). Thus, a wavelength absorbed by the conductive polymer is changed.
These conductive polymers absorb light in the visible region in the electrically neutral state and thus are colored. Oxidation of these conductive polymers allows wavelengths absorbed by the conductive polymers to shift to longer wavelengths.
In the case of the shift of the wavelengths to the infrared region, the polymers do not exhibit absorption in the visible region, so that the EC device is bleached.
Meanwhile, for the EC material containing the viologen-based compound described in PTL 2, dications are dissolved in the solution in a bleached state. Viologen is converted into radical cations by a reduction reaction, precipitated on the electrode, and colored.